The present invention relates to disc brakes for vehicles, and in particular to a system and method for mounting, removing and retaining brake pads in disc brakes, such as air-operated disc brakes utilized on commercial vehicles.
Pneumatically-operated disc brakes have been undergoing development and deployment on commercial vehicles since at least the 1970's, and are beginning to replace drum-style brakes due to advantages in areas such as cooling, fade resistance and serviceability. German Patent Publication No. DE 40 32 886 A1, and in particular FIG. 1 of this document, discloses an example of such an air disc brake. In this design, a pneumatic diaphragm chamber (pneumatic actuator) is attached to a rear face of the disc brake caliper housing, and applies a brake actuation force through a linear actuator rod to a brake actuator lever within the caliper. The brake's actuator lever in turn transfers and multiplies the force applied by the actuator rod to one or more spindles, which force brake pads against a brake disc or rotor. The terms “brake disc,” “rotor” and “brake rotor” are used interchangeably herein.
As shown in FIG. 1 of DE 40 32 886 A1, the actuator is located inboard of the brake caliper, in large part because commercial vehicle wheel rims are sized to only provide adequate clearance for the drum-type brakes historically employed on such vehicles. Because the resulting space envelope between the wheel and its axle is limited, the actuator must be located into the space adjacent to the wheel. For the same reason, brake pads must be configured to conform to the limited available radial space, and thus have typically been located and retained on one of the brake caliper or brake caliper carrier/mount using transverse suspension pins and/or using leaf spring-type metal strips disposed over the outer radius of the brake pads. Brake pads have also been retained by capturing the brake pads between the caliper mounting frame and the portion of the brake caliper which straddles the brake disc. (As one of ordinary skill will recognize, the same brake pad support function may be provided by a brake caliper carrier/mount designed to support the brake pads or by a brake pad carrier which is separate from the caliper mounting structure. For convenience in this description, the terms caliper carrier, caliper mount and brake pad carrier may be interchanged without intending to limit the brake par supporting structure to any specific brake pad and brake caliper carrying structure.)
Conventional commercial vehicle air disc brakes have typically required the installation of ancillary brake pad retention mechanisms, and/or use of the brake caliper itself to retain the brake pads during service. Both of these approaches, and in particular use of the brake caliper as the retention means, requires disassembly of the pad retention mechanism and/or removal of the brake caliper in order to replace worn brake pads and install new brake pads. As a result, brake pad replacement in previous air-operated disc brake designs has been a labor-intensive, and therefore costly, process.
A further problem with prior art brake pads is the tendency for the brake pad to rotate and/or vibrate during brake operation. As illustrated in FIG. 6, when a brake pad 101 is applied against a friction surface of a brake disc (not illustrated) which is rotating in direction DR, the brake disc's rotation induces motion and reaction forces between the brake pad 101 and its adjacent mount abutment surfaces (not illustrated for clarity). Specifically, at the leading edge 102 of the brake pad the brake pad attempts to move upward in direction LU in response to the friction forces along the face of the brake pad (illustrated here by force arrows across the face of brake pad 101). At the trailing edge 103 of the brake pad, the brake pad attempts to move downward in direction TD. However, because the brake pad 101 is constrained by adjacent mount abutment surfaces, the overall motion of the brake pad is generally a rotation about an axis parallel to the brake disc rotation axis. This motion may be unilateral during the brake application, or may manifest itself as a moderate-to-severe oscillation of the brake pad in its mount, significantly increasing wear of the abutting brake pad and mount surfaces.
In order to prevent undesired rotation and/or vibration of the brake pad within its mounting (for example, rotation about the brake application direction), the brake pad backing plate and the adjacent mounting bracket horns supporting the brake pads in the circumferential direction required a relatively tall radial height to minimize the amount of brake pad rotation before a corner of the backing plate contacted the adjacent mount horn (a motion referred to as “pad kick” or “pad turnout”). This relatively tall structure in turn would require the brake caliper, which is installed over the brake pads and mounting bracket, to have its corresponding opposing inner surfaces radially outboard of the mount horns be relieved enough to accommodate the outer corners of the brake pad and/or mount horns. A problem with this thinning is that because the maximum outer radius of the brake caliper is typically constrained by very tight clearance to the inside of the adjacent wheel rim, the brake caliper arms straddling the brake disc between the application side and the reaction side of the caliper may end up being thinner than desired in this region in order to accommodate both the tall carrier mount horn and the close-fitting wheel rim. This can lead to very high tensile and bending stresses in the thin region, and an undesired reduction in fatigue life and service life.
The present invention addresses these and other problems by providing a brake pad mounting and retention arrangement and method of installation and removal which provides greater ease in in-situ brake pad installation and removal, without brake caliper removal or other significant brake disassembly work. This solution is of particular significant in highly space-constrained commercial vehicle air disc brake applications, where brake pad servicing without significant brake disassembly work caliper was not previously believed to be commercially practical.
In one embodiment of the present invention the brake caliper mount pad support horns are provided with a narrow vertical groove or slot on the side of the horns facing away from the brake disc. This groove is arranged to permit the brake pad, having features of corresponding thickness one its lateral sides, to slide in the radially-inward direction down the groove, until the brake pad reaches the installed position. The horns further have lateral grooves at or near their bases extending parallel to the brake disc rotation axis, arranged to receive the side features of the brake pad, such that the brake pad may advance toward the brake disc when pushed forward by the brake caliper's actuator. During service, the brake pad is positively retained in the brake by the combination of: (i) the lateral grooves in the mount horns, preventing lifting of the brake pad out of the caliper and undue rotation of the brake pad (i.e., twisting or “tipping” of the pad in the caliper as the rotating brake disc attempts to raise one end of the brake pad while pushing down on the other end of the brake pad); (ii) the brake disc, which prevents the brake pad from advancing so far as to emerge from the mount horn lateral grooves; and (iii) by the brake application device, which prevents the brake pad from retreating so far as to reach the mount horn vertical grooves and slide out the top of the caliper.
This arrangement also facilitates easy and rapid brake pad changes, as all that is required to remove the brake pads after wheel removal is to fully withdraw the brake application device, slide the brake pad backing plate axially to reach the mount horn vertical groove, and then simply lift the brake pad out of an opening in the top of the still-installed brake caliper. A new brake pad may then be inserted into the vertical groove until the backing plate's lateral features are aligned with the lateral grooves, followed by advancing the brake application device a sufficient distance behind the brake pad to prevent its backing out of the lateral grooves.
The present invention also provides the ability to significantly lower the height of the mount horns, thereby permitting the brake caliper thickness in high-stress regions to be made thicker to increase strength and caliper service life. The geometry of the relatively compact grooves and corresponding engaging features on the brake pad backing plates ensures the amount of brake pad rotation within the carrier is significantly limited as compared to prior art arrangements. Accordingly, because there is no longer a need to provide relatively tall mount horns to limit pad rotation (as the grooves now limit pad rotation), the horns may be made shorter. With the reduction in horn height, the newly-found additional clearance between the horn and the inner surface of the brake caliper in the regions over the horns allows the caliper to be made thicker in these highly-loaded areas. The addition of caliper material in these regions provides additional load-bearing cross-sectional area, with corresponding reduction in local stress levels and increase in caliper fatigue life.
Preferably the brake pad, mount and/or brake caliper may be provided with vibration-damping features, such as spring-loaded brackets on the backing plate lateral extensions or a spring member engaging tabs on the upper surface of the backing plate and on the caliper or caliper mount.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.